The present invention relates to a method of correcting, in a laser printer, digital copier or similar image forming apparatus, a position for writing an image on a photoconductive element, and a device therefor.
A laser printer, for example, includes an optical writing unit for writing an image with a laser beam, and an image forming unit for forming an image. Usually, the writing unit is made up of a semiconductor laser, a collimator lens for collimating a laser beam issuing from the laser, a polygonal mirror provided with a mirror on each side of a regular polyhedron and rotated at high speed by a motor for steering the laser beam incident thereon, an f-theta lens for correcting the laser beam steered at an equal angular pitch by the polygonal mirror such that the beam scans the surface of a photoconductive drum at an equal linear pitch, a mirror for reflecting the laser beam onto the drum, a dust glass for preventing dust and other impurities from entering the writing unit, a synchronization sensor for sensing a position where a writing operation begins in the main scanning direction and implemented by, for example, a PIN photodiode, and a mirror for reflecting the laser beam toward the sensor. On the other hand, the image forming unit is made up of the photoconductive drum, a register roller for driving a recording medium, e.g., paper toward the drum such that the leading edge of the paper meets that of an image formed on the drum, a main charger for uniformly charging the surface of the drum by corona discharge, a developing unit for developing the latent image by a toner stored therein, a transfer charger for transferring the resulting toner image from the drum to the paper by corona discharge, a transport belt for transporting the paper carrying the toner image to a fixing section, a cleaning unit for removing the toner remaining on the drum after the image transfer with a cleaning blade, a fixing roller accommodating heater therein and having the surface thereof made of a heat-resistive and highly separable material and rotatable while being heated to a predetermined temperature, a pressure roller pressed against the fixing roller by a predetermined pressure, a paper cassette loaded with a stack of papers, a pick-up roller for feeding the papers one by one from the cassette, and a discharge roller for driving the paper having the toner image thereof fixed by the two rollers out of the printer to a tray.
A bicolor image forming apparatus is also extensively used. For example, in a conventional bicolor laser printer, laser beams issuing from a semiconductor laser and associated with image signals of respective colors are each steered in a particular direction by the polygonal mirror to scan the drum. Specifically, the laser beam associated with a first color is optically corrected by the f-theta lens and then directed toward the drum by the mirror. As a result, a latent image is electrostatically formed on the surface of the drum. A first developing unit develops the latent image by a toner of first color. Subsequently, the laser beam associated with a second color is incident to the drum to form an electrostatic latent image. Then, a second developing unit develops this latent image by a toner of second color.
Japanese Patent Laid-Open Publication (Kokai) No. 300259/1988 discloses an image forming apparatus comprising an image carrier for forming thereon, moving means movable for transferring the image from the image carrier at an image transfer station, means for sensing a register mark forming area provided on the moving means and implemented by a transparent body and a register mark formed in the register mark forming area, and correcting means for correcting the position of an image on the image carrier in response to an output signal of the sensing means. The sensing means is constituted by a mono or bidimensional imaging device, e.g., CCDs (Charge Coupled Devices) or MOS (Metal Oxide Semiconductor). Japanese Patent Laid-Open Publication No. 141746/1989 teaches a register mark reading device applicable to an image forming apparatus of the type having a plurality of image carriers and sequentially transferring images formed on the image carriers one above another to a paper being transported. The register mark reading device includes mark reading means for reading a main scan register mark and a subscan register mark formed on each image carrier and transferred to a movable body. Specifically, the mark reading means is implemented as a mark sensor in the form of CCDs. The mark sensor is linearly arranged to intersect the main scan and subscan register marks at a predetermined angle.
However, the problem with the conventional laser printer is that the relative position between the drum and the optical writing unit changes due to changes in the temperature inside the printer. At the same time, the positions of the laser, polygonal mirror, f-theta lens and mirror arranged in the writing unit change relative to one another due to the temperature changes. Such changes in position directly translate into changes in the positions of beam spots on the drum as well as changes in the writing width available with the beam. As a result, the images are dislocated and/or changed in width on the paper, failing to achieve desirable quality.
The conventional bicolor laser printer also has the above-described problem, i.e., the images of respective colors are brought out of register on the drum.
To eliminate the above problem, the image forming apparatus and the register mark reading device disclosed in the previously stated Laid-Open Publications each uses CCDs for sensing a toner This, however, brings about image. another problem that an exclusive drive circuit for the CCDs is needed, obstructing the miniaturization of the apparatus and increasing the cost.